Calibration strategies for the direct determination of Ca, K, and Mg in commercial samples of powdered milk and solid dietary supplements using laser-induced breakdown spectroscopy (LIBS)
Graphical abstract
Introduction
An equilibrated and healthy diet includes a large portion of chemical components essential for the proper physiological functions of the human body. Mineral elements, as Ca, K, Mg, and Na (macro elements), Cr, Cu, Fe, Mn, Se, and Zn (micro and trace elements) are generally found in a wide range of foods (Belitz, Grosch, & Schieberle, 2009). Calcium is involved in several processes, such as blood coagulation, muscular contraction and bone formation (Allgrove, 2003). Magnesium is a cofactor of enzymatic reactions (Barbagallo & Dominguez, 2007), and K participates in intracellular osmolality (Ekmekcioglu, Elmadfa, Meyer, & Moeslinger, 2016). A daily dose of these elements is important for the human body. On the other hand, this does not always occur, then fortified foods as powdered milk and solid dietary supplements can become an option, due to the special needs of some people, such as pregnancy and children with a restrictive diet. Powdered milk and solid dietary supplements contain a range of essential elements responsible for the health and child growth, being necessary a fast and reliable analytical method for quality control.
Traditional analytical techniques, such as inductively coupled plasma optical emission spectrometry (ICP OES), flame atomic absorption spectrometry (FAAS) and ICP-mass spectrometry (ICP-MS) have been applied for the quantification of several elements, but generally require a pre-treatment to convert the solid sample to an aqueous homogeneous solution. During the analytical sequence, errors can be introduced due to the manipulation that reduces analytical frequency and generate residues (Chinni, Cremers, & Multari, 2010). Other analytical strategies include analysis of samples as suspension using ICP OES or FAAS (Asfaw and Wibetoe, 2005, Sola-Larrañaga and Navarro-Blasco, 2009).
Analytical techniques such as laser-induced breakdown spectroscopy (LIBS) has been used in several applications related to food samples, like classification of red wine (Moncayo, Rosales, Izquierdo-Hornillos, Anzano, & Caceres, 2016), identification of meat species (Bilge, Velioglu, Sezer, Eseller, & Boyaci, 2016), and boron determination in meatballs (Hedwig et al., 2016). For those studies, LIBS advantages include: direct analysis with minimal or no sample preparation, high analytical frequency, the use of a small quantities of sample (typically < 100 mg), and multielement capability (advantage if compared with FAAS) (Pasquini, Cortez, Silva, & Gonzaga, 2007). Spectra obtained from LIBS technique present several emission lines allowing a fast sample inspection and the application of chemometrics tools for calibration and classification purposes (Neiva, Chagas Jacinto, Mello de Alencar, Esteves, & Pereira-Filho, 2016). For calibration, several regression models, employing univariate or even, multivariate analysis are possible, using for instance partial least squares (PLS) (El Haddad et al., 2014, Hernández-García et al., 2017).
On the other hand, disadvantages related to method calibration are observed, because the ablation process involves some μg of samples and it is not available reference material with certified values concentration for masses in the range of μg or ng, for example (Andrade and Pereira-Filho, 2016, Andrade et al., 2016, Augusto et al., 2016). In addition, direct analysis presented some difficulties in solid analysis, such as the reproduction of the data related to the process of ablation, formation of the plasma, microheterogeneity and matrix effects. These issues can compromise quantitative analysis (Mukhono, Angeyo, Dehayem-Kamadjeu, & Kaduki, 2013).
The goal of this study is to present a simple and fast method for the direct analysis of powdered samples of milk and solid dietary supplements using LIBS and determine the contents of three major constituents: Ca, Mg and K. Univariate and multivariate analysis strategies were tested and combined to build regression models. Some procedures, for instance sample dilution, were used to minimize matrix effects and achieve results with satisfactory accuracy, when solid samples are directly measured.
Section snippets
Reagents, sample description and preparation for ICP OES determinations
All reagents were of analytical grade or higher purity. Deionized water (18.2 ΩM cm resistivity) produced by a Milli-Q® Plus Total Water System (Millipore Corp., Bedford, MA, USA) was used to prepare all of the solutions. Prior to use, all glassware and polypropylene flasks were washed with soap, soaked in 10% v/v HNO3 for 24 h, rinsed with deionized water and dried to ensure that no contamination occurred. The multi-element standard solutions were prepared daily from 10,000 mg L− 1 Ca along with 1000
ICP OES reference analytical method
As mentioned before, a digestion of all the samples (powered milk and solid dietary supplements) was performed and the solutions were analysed by ICP OES to obtain reference values for Ca, K and Mg. Table 1 presents the emission lines studied in ICP OES instrument in axial and radial views. The emission lines that presented concordant results between axial e radial views were selected and the average of the concentrations (n = 3) results was considered as reference values. Table 4 shows the
Conclusions
The proposed method is suitable, fast and can be implemented for the direct determination of Ca, K, and Mg in solid food samples. In the case of Ca, limitations from matrix interference were minimized after dilution of the material using cellulose, case of the supplement samples. It was also concluded that the normalization process of the raw data plays an important role in the quality of the results.
Acknowledgement
This study was supported by the São Paulo Research Foundation (FAPESP), 2014/11415-0 PhD grant to ASA and 2016/00779-6, the National Council for Scientific and Technological Development (CNPq, 401074/2014-5, 305637/2015-0 and 445729/2014-7) and Pró-reitoria de Pesquisa (PROPe-UNESP, Grant 0268/001/14). The authors are grateful to Analítica and Thermo Scientific for the loan of instruments.
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